It has been observed that bacteria in both single culture and mixed cultures are likely to derive significant benefit from the ability to co-ordinate their population dynamics (Shapiro, 1988). ‘Quorum sensing’ as this mechanism is known is the ability of bacteria to link gene expression with population density. Signals produced by the organism are expressed into their environment and upon critical quorum signalling then activate a response regulator. This allows single cells to interact with others of their same and different species. In this manner, bacteria can coordinate their production of defence chemicals, differentiation, reproduction and migration.
Hitherto, by far the most studied applications of the quorum sensing signal compounds has been in the preparation of diagnostics and the stimulation in vitro of otherwise transiently available antibiotic compounds. It is now recognised that many bacterial species utilise this signal transduction process by means of a small range of simple molecules serving as autoinducers of virulence and other characteristics. The first molecule to be identified was N-(3-oxohexanoyl) homoserine lactone (OHHL) as the inducer of bioluminescence in Vibrio fisheri in 1981 (Eberhard et al, 1981).
In Vibrio species, OHHL production is reliant on density dependent lux gene transcription activated by a protein LuxR. The product Lux1 binds with LuxR and OHHL to become activated. This then is the general model process for the coordination of various phenotype expression. It was subsequently found that OHHL was part of a group of compounds, the acyl homoserine lactones (AHLs), many of which (both natural and synthetic) have signalling ability. Other molecules that are known to be quorum sensing signals or autoinducers include various peptides such as ‘competence signalling peptide’ in Bacillus subtilis and Streptococcus pneumoniae. 
Of the AHLs, N-(3-hydroxybutanoyl) homoserine lactone; N-hexanoyl homoserine lactone; N-butanoyl homoserine lactone; N-(3-oxooctanyl) homoserine lactone; N-octanoyl homoserine lactone; N-(3-oxodecanoyl) homo serine lactone; N-octanoyl homoserine lactone; 7,8-cis-N-(3-hydroxytetradecanoyl) homoserine lactone and other analogues have also been shown to be active. Other quorum sensing signals are known to be utilised by certain organisms including 3-hydroxypalmitic acid methyl ester.
The AHLs appear to be utilised only in the Gram negative bacteria, while Gram positive bacteria appear to use thiolactone peptide signalling molecules and other oligopeptides fragments for cell signalling.
The manipulation of the rumen and gut microbiology has hitherto been accomplished using antibiotics, including anti-microbials such as virginiamycin. In the past, many different types of natural and artificial compounds, including sulphonamides, tetracyclines and penicillin have been used. Their primary function has been to modify the rumen microbial populations in such a way as to reduce the undesirable bacteria and favour the beneficial bacteria.
However, there is increasing concern about the long term consequences of the use of these compounds in sub-therapeutic concentrations with fears that resistance to these drugs may now be widespread.
The complex work done in the rumen reticulum to convert cellulose, hemicellulose and lignins into available energy, while at the same time providing the host animal with on-going source of protein, is achieved by a community of bacterial species. This community is intensely competitive. Low methane producing rumen systems that are good producers of propionates are better at delivering energy to the animal. The optimisation of the rumen to this end has been the enduring target of drug and nutritional intervention. The major aim therefore is to maximise microbial protein production and cellulose/lignin-type compound degradation, while minimising negative aspects of undesirable microbial growth. The organisms that consume or degrade protein and increase (energy consuming) methane production are themselves non-mutualistic in their relationship with the host or deprive the animal of available starch and are therefore considered deleterious to optimal rumen fermentation.
The difficulty in using feed additives or other bioactive treatments is that of specificity as many substances have multifold effects on the microbial community. Reduction of proteolysis and deamination activity is partly responsible for increased performance of the animal. The control of the microbial population can also positively influence the production of volatile fatty acids and reduce methane production. The combined manipulation of these parameters results in improved animal performance, sometimes by very substantial margins.
WO97/27851 (The Johns-Hopkins University) discloses that the growth of Mycobacteria can be inhibited through the administration of homoserine or a homoserine lactone. The application suggests that these compounds are used in the diagnosis and treatment of M. tuberculosis infection in humans.
U.S. Pat. No. 5,591,872 (University of Iowa) discloses that N-(3-oxododecanoyl) homoserine lactone is an autoinducer which regulates gene expression in Pseudomonas aeruginosa and says that analogs or inhibitors of this autoinducer can be used in treating or preventing infection by this microorganism.
WO01/74801 (University of Nottingham) discloses a family of N-acyl homoserine lactones and their use as immunosuppressants.